In 1965, Melzack and Wall described the physiologic mechanisms by which stimulation of large diameter non-pain sensory nerves could reduce the amount of unpleasant activity carried by pain nerves. This landmark observation published in Science was termed the "gate control theory" and offered a model to describe the interactions between various types of the sensory pathways in the peripheral and central nervous systems. The model described how non-painful sensory input such as mild electrical stimulation could reduce or gate the amount of nociceptive (painful) input that reached the central nervous system. The gate-control theory stimulated research that lead to the creation of new medical devices such as transcutaneous electrical nerve stimulators (TENS). In brief, TENS works by electrically "blocking" pain impulses carried by peripheral nerves. TENS is widely used and endorsed by the pain management guidelines of both the AHCPR and American Geriatric Society (Gloth 2001). However a significant number of patients fail to achieve adequate relief with TENS or fail within six months of starting treatment (Fishbain et al., 1996). This project addresses a major clinical challenge and a large commercial opportunity, how to improve the analgesic effectiveness of TENS. Studies indicate that both heat (Abeln et al, 2002 and Weingand et al, 2001) and vibration (Lundeberg, 1983, 1984a, and Guieu 1990) also produce analgesia and may enhance TENS effectiveness. Advances in manufacturing, battery technology, miniaturization, and microchip processing now allow for the creation of a small portable patient controlled heating device that can be used with commercial TENS devices. This project will produce such a device and examine whether heat delivered by the device increases the effectiveness of TENS. The primary hypothesis is that heat will increase the analgesic effectiveness of TENS in subjects who suffer from myofascial low back pain. The secondary hypothesis is that heat is as effective as TENS in providing pain relief in these subjects.